The Autoimmune Lymphoproliferative Syndrome is an inherited disease associated with a defect of lymphocyte apoptosis, lymphoproliferation and autoimmunity. Although, there are treatments for many of its autoimmune complications, there currently is no safe and effective therapy for this syndrome itself. Recently investigators in Europe serendipitously found that a child with ALPS type I had significant clinical improvement while on pyrimethamine/sulfadoxine (Fansidar) for Pneumocystis carinii prophylaxis.